How to crop an image that is zoomed(zoomed in/out)

How to crop an image that is zoomed(zoomed in/out). Cropping works fine when the image is normal, that is not zoomed. I have used a panel whose AutoScroll property is true and a picturebox inside the panel whose sizemode property is AutoSize and BackgroundImageLayout is Tile.

Load image:
Image img = Image.FromFile("filepath");
picBoxImageProcessing.Image = img;

zoom in:

zoomFactor += 1;
picBoxImageProcessing.Size = new Size((img.Width * zoomFactor), (img.Height * zoomFactor));
picBoxImageProcessing.SizeMode = PictureBoxSizeMode.StretchImage;

cropping:
Draw a rectangular shape on the image and then crop.
Crop(Image img, Rectangle r)
{
//
}

In this way when i zoom in first and then select a specific region drawing a rectangle on the zoomed image and then crop then another region is cropped rather than i have selected.
I have got many application for image processing but I couldn't solve my problem.

thanks in advance.